Police entered the campus encampment early Tuesday, WLS reported.
Protests have broken out at colleges and universities across the country in connection with the war in Gaza.
Many pro-Palestinian protesters are calling for their colleges to divest of funds from Israeli military operations, while some Jewish students on the campuses have called the protests antisemitic and said they are scared for their safety.
The student protests -- some of which have turned into around-the-clock encampments -- have erupted throughout the nation following arrests and student removals at Columbia University in New York City. Students at schools including Yale University, New York University, Harvard University, University of Texas at Austin, University of Southern California and more have launched protests.
There have been about four dozen arrests so far by the NYPD at Columbia University, according to police sources.
The first bus of protesters apprehended by police just left the campus area.
-ABC News' Aaron Katersky
Police officers were met with a heavily fortified building when they entered Hamilton Hall on Tuesday night, sources told ABC News.
There have been a number of arrests, however no specific number was immediately available.
With the situation at Hamilton Hall believed to be contained, a number of officers are making their way from building to tent encampments.
-ABC News' Aaron Katersky
Columbia University has asked the NYPD to "retain a presence on campus through at least May 17."
The request came in the letter the university sent to the NYPD Tuesday night, allowing them to move onto campus grounds.
-ABC News' Aaron Katersky and Joshua Hoyos
Columbia University has released a statement addressing its authorization to allow the NYPD on campus, saying the decision was "made to restore safety and order to our community."
In its statement, the university said after they learned protesters had entered Hamilton Hall last night and "occupied, vandalized, and blockaded" it, they were left with "no choice" but to allow the police in. The university also said in a statement that they believe "that the group that broke into and occupied the building is led by individuals who are not affiliated with the University."
"Columbia public safety personnel were forced out of the building, and a member of our facilities team was threatened. We will not risk the safety of our community or the potential for further escalation," the university's statement continued.
The university said in its statement the NYPD's presence is about the "actions of the protestors, not the cause they are championing."
"We have made it clear that the life of campus cannot be endlessly interrupted by protesters who violate the rules and the law," the statement read.
-ABC News' Joshua Hoyos
